In a conventional technique, an X-ray diagnosis apparatus has a configuration in which a subject is irradiated with an X-ray, and an X-ray image of the subject is displayed on a monitor, etc. based on X-ray image data outputted from an X-ray detector according to the X-ray incident on the X-ray detector after passing through the subject.
The X-ray detector has variations such as an image intensifier and an X-ray flat panel detector. As for the X-ray flat panel detector, an important technology is to eliminate an offset component of a detected element channel which changes over time.
For instance, an indirect X-ray flat panel detector disclosed by U.S. Pat. No. 4,689,487 is comprised of a scintillator (e.g. using cesium iodide (CsI)) for converting an incident X-ray to light and an element for converting the light outputted from the scintillator to electric charge (e.g. a photodiode using amorphous silicon (a-Si)). In addition, a direct X-ray flat panel detector disclosed by U.S. Pat. No. 5,319,206 is comprised of a conversion element using a substance (e.g. selenium (Se) or lead iodide (PbI)) for directly converting the incident X-ray to electric charge.
In either type of X-ray flat panel detector, electric charge outputted from the conversion element constituting each pixel is read out as an image signal through a switching element such as a thint film transistor (TFT). As there is one conversion element per one pixel in the X-ray flat panel detector, it is necessary, in the case where the X-ray flat panel detector has the pixels of 1000 columns×1000 rows, to acquire image data by reading the image signal from a million conversion elements. Thus, to speed up image data acquisition, the pixels are divided into a plurality of pixel groups and readout channels corresponding to the respective pixel groups are provided to perform readout operation in parallel so as to output the image data from each readout channel.
As an offset for each readout channel of the X-ray flat panel detector is different from one another, it is necessary to acquire offset data for each readout channel in advance and use that offset data upon image taking to correct the image data individually for each readout channel.
The X-ray diagnosis apparatus has a radiography mode for reading the image signals individually from all the conversion elements of the X-ray flat panel detector and obtaining a shot image of high resolution (of a large number of pixels), and additionally has a fluoroscopy mode for obtaining a fluoroscopic image of low resolution (of a small number of pixels) at an image rate of 30 images per second for instance. In this fluoroscopy mode, the subject is continuously irradiated with the X-ray and so an X-ray dosage is kept low. Therefore, to improve a signal-to-noise (SN) ratio, the conversion elements are divided into groups (for example, one group is comprised of 2×2 elements) and electric charges obtained from the conversion elements for each group are added up for reading out the image signals.
The offset data are prepared for each of the radiography mode and the fluoroscopy mode so that the offset data used for offset correction is switched according to the mode.
The offset data changes according to a temperature characteristic of a readout circuit, and so it needs to be periodically updated.
As disclosed in Japanese Patent Application Publication No. 7-72254 and Japanese Patent Application Publication No. 2002-204793 (corresponding to U.S. unexamined Patent Publication 2002/0064254 A1), in the case of updating the offset data, a period for not irradiating the X-ray flat panel detector with the X-ray is provided, an arithmetic mean is taken to offset image data of a plurality of frames obtained from the X-ray flat panel detector for each readout channel according to the mode so as to calculate new offset data to be updated.
In an inspection for alternately repeating a plurality of modes such as fluoroscopy and radiography, it is necessary to provide a period without irradiation of X-ray during inspection in order to alternately acquire the offset data. As for a system using the photodiode, there are the cases where an afterimage exists after finishing X-ray irradiation and the offset data cannot be collected until the afterimage is reduced so that the period incapable of X-ray irradiation is extended.
In the case of performing a catheter operation and so on in the fluoroscopic mode, there is the period without X-ray irradiation due to change of a catheter, preparations for a contrast agent and so on so that it is possible to collect the offset data in the fluoroscopy mode in that period. However, there is a problem that it is not possible to secure the period of no X-ray irradiation in the radiography mode and it is difficult to acquire the offset data in the radiography mode because the radiography is performed by shifting from the fluoroscopy mode to the radiography mode immediately (in one to two seconds ordinarily) and it moves from the radiography mode to the fluoroscopy mode immediately after finishing the radiography to check a catheter status.
Furthermore, incident dosage is large enough in the radiography mode to reduce influence on an image caused by variation of an offset less than that in the fluoroscopy mode. In the case where offset data are not acquired for a long time, artifacts may be generated in the image. The change in the offset may significantly influence image quality in the case of adopting the mode for obtaining the fluoroscopic image (partial region fluoroscopy mode) by using the pixels of the radiography mode while keeping the resolution and limiting a field of view without adding pixels.
Moreover, the offset data may be collected by each pixel in place of each readout channel of the X-ray flat panel detector. In this case, there is a problem that collection of the offset data is time-consuming, and in particular, it is more time-consuming in the radiography mode than in the fluoroscopy mode because of a large number of pixels in the radiography image.
The present invention is provided in consideration of such circumstances, and an object thereof is to provide an X-ray diagnosis apparatus capable of acquiring offset data in another mode from the offset data acquired in one of a plurality of modes, and performing a precise offset correction equivalent to newly acquiring offset data in all the modes.